Recently, various forms of Code-Division Multiple Access (CDMA) wireless communication systems have become standardized and deployed in a number of countries. More specifically, such standards include IS-95, cdma2000 and WCDMA. In CDMA systems, users are distinguished from each other by being assigned different code sequences.
Within CDMA systems, a “pilot” signal is often transmitted by a transmitter in order to facilitate the performance of a number of operations at a receiver. For example, the pilot signal may be utilized by the receiver to establish timing and frequency synchronization with the transmitter, to estimate channel characteristics, and to facilitate coherent demodulation of transmitted data. Each pilot signal is typically uniquely associated with a particular base station in order to enable a mobile station to identify the base station from which a pilot signal is received. This association may be achieved by assigning each base station a different offset to employ when generating its PN sequence. Generation of a pilot signal is typically effected by “covering” a known data pattern with a known Walsh code and spreading the result with a known pseudo-noise (PN) sequence.
Rake receivers operative to implement diversity reception techniques are often deployed within CDMA systems to extract pilot, user traffic and other data from signal energy received over a channel. Each “finger” or diversity branch of the Rake receiver processes its assigned multipath component and recovers the associated pilot signal, user traffic and other data. Recovered pilot signal information is generally used to estimate the characteristics of the channel (e.g., amplitude and phase) needed for coherent detection within the Rake receiver. Typically, the pilot signal is generated so as to be orthogonal to the spreading codes assigned to system users so as not to engender interference during the data demodulation process. However, in the case of dispersive multipath channels, the various received multipath signal components will be mutually non-orthogonal and will hence create undesirable interference. Since the contribution of the pilot signal to the transmitted signal is generally somewhat substantial, multipath interference arising from non-orthogonal pilot components can degrade receiver performance.
Accordingly, a need exists for a technique for canceling multipath pilot signal interference within diversity receivers.